EP3509739A1

EP3509739A1 - Adsorbent comprising layered double hydroxide and activated carbon

Info

Publication number: EP3509739A1
Application number: EP17752153.1A
Authority: EP
Inventors: Amitava Pramanik; Somnath Das; Priyanka SARKAR; Shanthi APPAVOO
Original assignee: Unilever PLC; Unilever NV
Current assignee: Unilever PLC; Unilever NV
Priority date: 2016-09-06
Filing date: 2017-08-21
Publication date: 2019-07-17
Also published as: CN109689201A; WO2018046286A1

Abstract

The present invention relates to a material that absorbs volatile compounds, in particular sulphurous compounds. It is found that an absorbent of high surface area suitable for highly volatile compounds can be obtained by growing nanoparticles of a layered double hydroxide (LDH) on the surface of powder activated carbon (PAC). Disclosed is a composite adsorbent comprising nanoparticles of a layered double hydroxide (LDH) and a powder activated carbon (PAC) of the general formula [M²⁺ _1-xM³⁺ _x (OH)₂]^q+(X^n-)_q/n.yH₂O, in which, M²⁺ = Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, CO²⁺, Ni²⁺, Cu²⁺ or Zn²⁺; M³⁺ = Al³⁺ or Fe³⁺; x = [M³⁺]/ [M³⁺ + M²⁺]; q = x n = 1 to 4; and, y = number of water molecules present on the LDH; X = anions selected from halides, sulphates, nitrates, carbonates or anionic moiety from organic sources wherein said composite is obtainable by a hydrothermal process comprising, in sequence, the steps of: (a) contacting, and mixing, powder activated carbon with a water soluble salt of M²⁺ and a water soluble salt of M³⁺; (b) adding an alkali to the dispersion of step (a); (c) aging the dispersion of step (b) by heating it to 80 to 100 °C; (d) separating, by any means, the aqueous phase of the dispersion from the dispersed phase; and (e) washing said dispersed phase with water to remove excess alkali, to get the composite adsorbent, wherein amount of nanoparticles of said layered double hydroxide (LDH) is 10 to 30% by weight and wherein particle size of said powder activated carbon is 50 μm to 500 μm.

Description

ADSORBENT COMPRISING LAYERED DOUBLE HYDROXIDE AND ACTIVATED

CARBON

Field of the invention

The present invention relates to adsorbents. In particular, the present invention relates to composite absorbents suitable for adsorption of volatile sulphurous compounds.

Background of the invention

Activated carbon and layered double hydroxide (LDH) are widely used as adsorbents, albeit for adsorbing different kinds of materials. Controlling the kinetics of adsorption and the release of highly volatile compounds is a technical problem. Carbon, being one of the high surface area materials, is known for its ability to adsorb and release volatile compounds. However, an admixture of carbon and metallic hydroxides/ oxides or other inorganic adsorbents do not show significantly enhanced adsorption characteristics. It is difficult to synthesize such compounds by a chemically benign route and template synthesis often results in higher cost of the products but lower yields. Layered double hydroxides (LDH) are usually synthesized by co-precipitation process. In such process, an aqueous solution of metal ions and an alkali are mixed in a pool of water under stirring. The reaction is sometimes carried out under elevated temperature to improve crystallinity and size of the particles. Nevertheless, the particles synthesized by this process have significantly larger size, usually micron size.

US4458030 A (KYOWA KAGAKU KOGYO KK, 1984) discloses an adsorbent composition comprising a combination a hydrotalcite and activated carbon. The composition adsorbs variety of substances. The adsorption capacity is not an additive effect of the capacity of the individual ingredients. The activated carbon may be powdered or granular. However, the composite is produced by admixture of the ingredients by mixing the powders with a suitable binder such as water and polyvinyl alcohol, and the mixture is kneaded and molded. The mixture can be molded into any desired shape such as hollow cylinder, sphere, solid cylinder or polygonal prism having diameter of 1 to 50 mm, and it is thereafter dried and calcined.

US2013/0316898 AA (THE UNIVERSITY OF SURREY) discloses a composite adsorbent material, and in particular, a highly porous carbon-based composite material for adsorption and stabilisation of inorganic substances. The composite adsorbent material comprises a porous carbon carrier matrix and an adsorbent species, wherein the adsorbent species is precipitated within the pores of the carrier matrix. The invention extends to various uses of such adsorbent materials, for example in water purification, recovery of metals from waste streams and remediation applications, and where the adsorbent material is amended into soil for the purpose of breaking pollutant-receptor linkages. However, there is no disclosure of adsorption of volatile compounds.

Although the cited article discloses absorbents for various types of materials; an absorbent for volatile compounds, especially sulphurous compounds, is still an unmet need.

Therefore, it is an object of the present invention to provide an absorbent for volatile compounds.

It is another object to provide an absorbent with very high surface area.

Surprisingly, we have determined that anabsorbent with very high surface area, which is suitable for volatile compounds, can be obtained by growing nanoparticles of layered double hydroxide (LDH) on the surface of powder activated carbon (PAC) by a hydrothermal process.

Summary of the invention Accordingly, in a first aspect, is disclosed a composite adsorbent comprising nanoparticles of a layered double hydroxide (LDH) and a powder activated carbon (PAC) of the general formula [M²⁺i_-xM³⁺x (OH)₂]^q+(Xⁿ-)_q/n-yH20, in which,

M²⁺= Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺; M³⁺ = Al³⁺ or Fe³⁺;

x = [M³⁺]/ [M³⁺ + M²⁺];

q = x

n = 1 to 4; and,

y = number of water molecules present on the LDH;

X= anions selected from halides, sulphates, nitrates, carbonates or anionic moiety from organic sources

wherein said composite is obtainable by a hydrothermal process comprising, in sequence, the steps of:

(a) contacting, and mixing, powder activated carbon with a water soluble salt of

M²⁺ and a water soluble salt of M³⁺;

(b) adding an alkali to the dispersion of step (a);

(c) aging the dispersion of step (b) by heating it to 80 to 100 °C;

(d) separating, by any means, the aqueous phase of the dispersion from the dispersed phase; and

(e) washing said dispersed phase with water to remove excess alkali, to get the composite adsorbent, wherein amount of nanoparticles of said layered double hydroxide (LDH) is 10 to 30% by weight and wherein particle size of said powder activated carbon is 50 μηη to 500 μηι.

In a second aspect is disclosed a hydrothermal process for preparing a composite adsorbent comprising nanoparticles of a layered double hydroxide (LDH) and a powder activated carbon (PAC) of the general formula [M²⁺i-_xM³⁺ _x (OH)₂] ⁺(X^n")_q/n-yH₂0, in which,

M²⁺= Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺;

M³⁺ = Al³⁺ or Fe³⁺;

x = [M³⁺]/ [M³⁺ + M²⁺];

q = x

n = 1 to 4; and,

y = number of water molecules present on the LDH;

X= anions selected from halides, sulphates, nitrates, carbonates or anionic moiety from organic sources comprising, in sequence, the steps of:

(a) contacting, and mixing, powder activated carbon with a water soluble salt of M²⁺ and a water soluble salt of M³⁺;

(b) adding an alkali to the dispersion of step (a);

(c) aging the dispersion of step (b) by heating it to 80 to 100 °C;

In accordance with a third aspect is disclosed a composite adsorbent consisting of

5 to 30 % by weight of layered double hydroxide (LDH) nanoparticles and 70 to 95 % by weight powder activated carbon (PAC), where general formula of said LDH is

[M²⁺i_-xM³⁺x (OH)2]^q+(Xⁿ-)_q/n-yH₂0, in which:

M³⁺ = Al³⁺ or Fe³⁺;

x = [M³⁺]/ [M³⁺+M²⁺];

q = x

n = 1 to 4;

y = number of water molecules present on LDH,

wherein amount of nanoparticles of said layered double hydroxide (LDH) is 10 to 30% by weight and wherein particle size of said powder activated carbon is 50 μηη to 500 μηι.

These and other aspects, features and advantages will become apparent to those of ordinary skill in the art from a reading of the following detailed description and the appended claims. For the avoidance of doubt, any feature of one aspect of the present invention may be utilised in any other aspect of the invention. The word "comprising" is intended to mean "including" but not necessarily "consisting of" or "composed of." In other words, the listed steps or options need not be exhaustive. It is noted that the examples given in the description below are intended to clarify the invention and are not intended to limit the invention to those examples per se. Similarly, all percentages are weight/weight percentages unless otherwise indicated. Except in the operating and comparative examples, or where otherwise explicitly indicated, all numbers in this description indicating amounts of material or conditions of reaction, physical properties of materials and/or use are to be understood as modified by the word "about".

Numerical ranges expressed in the format "from x to y" are understood to include x and y. When for a specific feature multiple preferred ranges are described in the format "from x to y", it is understood that all ranges combining the different endpoints are also contemplated.

Detailed description of the invention The Process

In accordance with a first aspect, the invention providesa composite adsorbent comprising nanoparticles of a layered double hydroxide (LDH) and a powder activated carbon (PAC), obtainable by a hydrothermal process.

Hydrothermal process involves the combined effect of water and pressure on the functionality of the crystals which get synthesized. Thus, it is pertinent to use aqueous systems for such synthesis. This process is generally preferred over other standard techniques for obtaining materials with high crystallinity and purity.

In the present invention, the hydrothermal process ensures that the particle size is significantly lower (in nanoscale dimension). However, in the absence of any spacer/ carrier, the nano particulate LDH is amenable to form cluster aggregates, which could eventually impact the effective surface area of the material. The powder activated carbon (PAC) acts as a carrier/ spacer on which the nanometer sized particles of LDH are grown by hydrothermal process. This ensures that aggregates are formed in minimal quantity. The process ensures uniform coverage of the LDH on the carbon particles, thereby resulting in a composite which has high affinity for odorous compounds, in particular sulphurous compounds.

The hydrothermal synthesis begins with contacting, and mixing, powder activated carbon with a water soluble salt of M²⁺ and a water soluble salt of M³⁺. Preferably, PAC is soaked in a solution of a mixture of a salt containing a trivalent metal ion and a salt containing a bivalent metal ion for at least four hours to ensure near complete adsorption of metal ions in the carbon matrix and the dispersion is further mixed for at least two hours, preferably using a sonicator. It is preferred that the molar ratio of the trivalent metal ion to the bivalent metal ion of the layered double hydroxide is 1 :2 to 1 :4, more preferably 1 :3.

A lower ratio between the amount of the liquor (i.e., water) to that of carbon, ensures effective adsorption of metal ions and reduces the chances of loss of ions in the bulk solution.

In step (b), an alkali is added to the dispersion of step (a).

A solution of an alkali is added to the dispersion until pH of the dispersion increases to 10 and then it is stirred vigorously for about fifteen minutes in a stirrer, preferably a magnetic stirrer at 800 to 1000 RPM till the mixture converts from gelatinous state to powdery dispersion in water. It is preferred that the alkali is a mixture of sodium hydroxide and sodium carbonate. It is further preferred that molarity of sodium hydroxide is about4M and that of sodium carbonate is 15M.

In the next step the dispersion of step (b) is aged by heating it to 80 to 100 °C, preferably for about 24 hours. Preferably the total volume of the aqueous mixture is maintained between 70 ml to 150 ml. Some residual head space is necessary for generation of water vapor which exerts pressure in the confined environment. Typically, in the absence of use of any blanketing gas like nitrogen, the pressure exerted by the water vapor in such a system at about 100 °C is between 1 and 2 bar. At the completion of the reaction, the aqueous phase of the dispersion is separated from the dispersed phase by any means such as simple filtration under vacuum using a filter medium like a cloth or filter paper; and the dispersed phase is washed thoroughly with water, preferably distilled, to remove excess alkali, to get the composite adsorbent. It is preferred that before employing any means of separation, the dispersion is cooled to room temperature (about 25 °C). The composite material thus obtained is dried in air oven, for example at 80 ^°C for 24 hours.

The composite may optionally be calcined.

Properties of the LDH, the carbon and the composite material

It is preferred that the bivalent metal of the LDH is Mg²⁺, Cu²⁺ or Zn²⁺, more preferably Mg²⁺ or Zn²⁺.

It is preferred that the amount of nanoparticles of the layered double hydroxide (LDH) is 10 to 20 % by weight of the composite material, more preferably 10% by weight of the composite material. Similarly, the amount of PAC is 70 to 90 % by weight of the composite material, preferably 80 to 90 % by weight, and most preferably 90 % by weight of the composite material.

It is preferred that pore size of the powder activated carbon (PAC) is 1000A to 2000A.

The particle size of the powder activated carbon is 50 μηη to 500 μηη, more preferably 50 m to 300 μηι.

It is preferred that surface area of the composite material is 750 to 1000 m²/g. The term surface area means the BET surface area which may be determined by any standard measurement technique known in the art.

It is particularly preferred that the composite adsorbent material contains 10% by weight layered double hydroxide and 90 % by weight powder activated carbon. The invention will now be further illustrated by means of the following non-limiting examples.

Examples

Materials

All reagents used were A.R. grade procured from Sigma Aldrich. Zinc nitrate hexahydrate [Zn(N03)2.6H20], aluminium nitrate nonahydrate [AI(N03)3.9H20], sodium hydroxide, anhydrous sodium carbonate, powder activated carbon (Supplier: Active Carbon Ltd, India; particle size 50 μηη to 300 μηη) and deionised water were used for the synthesis.

Method

Preparation of the composite adsorbent according to the invention:

PAC was soaked in 50 ml of an aqueous solution of a mixture of aluminium nitrate (0.033mole) and zinc nitrate (0.1 mole) at molar ratio of 3:1 , sonicated for four hours and allowed to stand overnight. This formed a dispersion.

The dispersion was transferred to a 250-ml stoppered bottle and an alkaline aqueous solution of a mixture of sodium hydroxide and sodium carbonate (4M sodium hydroxide and 1 .5M sodium carbonate was added until the pH reached 10. The dispersion was mixed vigorously for fifteen minutes under magnetic stirring at 800 to 1000 RPM.

The bottle was stoppered tightly to avoid leakage of air and the bottle was kept for 24 hours inside an air oven set at 85^°C. Total volume of the aqueous mixture inside the 250-ml stoppered bottle was maintained between 70 ml to 150 ml, leaving enough head space for generation of water vapor which exerted pressure the confined environment. At the completion of the reaction, the contents of the bottle were cooled to room temperature (25°C) and filtered using a Whatman® 42 filter paper under vacuum. The particles (of the dispersed phase) were washed thoroughly using distilled water. The composite material thus obtained was air dried at 80°C for 24 hours.

Some more composite adsorbent materials were synthesized by varying the proportions of PAC and LDH . All materials thus synthesized were subjected to tests in which HS-GC technique was used to determine the affinity of the materials towards volatile malodorous gases.

Preparation of an admixture:

A few admixtures (i.e., simple physical mixtures) of LDH and PAC were prepared in a typical reaction system in which 'x' g of synthesized LDH was mixed with (100-x) g of powder activated carbon. The value of x is from 10 to 40. The two were mixed by swirling assisted by a stirrer, followed by vortexing for about five minutes.

The mixtures were used for also were subjected to tests in which HS-GC tech was used to determine their affinity towards volatile malodorous gases.

The HS-GC technique:

A mixture of dimethyl disulphide, diethyl disulphide, isovaleric acid and valeric acid (2 L of each) was added to 8 μί of ethanol and the entire mixture was transferred to a 22 mL GC headspace vial which was sealed immediately. This was used as a reference standard.

The same concentration of the standard mix was pre-adsorbed on 50 mg of the inventive adsorbent materials made in accordance with the invention (as well as the admixtures which were outside the scope) and the concerned vials were also sealed immediately. All the vials were incubated at 40 ^°C for ten minutes and the headspace concentration of these actives were analyzed using GC with headspace sampler. The following headspace conditions were used: Capillary column: CPWAX 52CB (30 m length x 0.25mm ID)

Oven program: 40 °C - 10 °C /minute to 250°C - hold for 10 minute

Carrier gas: Helium (1 mL/minute)

Thermostat condition: Temperature 40 °C; Incubation time 10 minute

Needle temperature: 70 °C

Transfer line temperature: 1 10 °C

Example 1 : Comparison of efficacy of composites with admixtures

One aim of this experiment was to determine the affinity of composites made in accordance with the invention towards sulphur based volatile malodorous compounds (by adsorption). Another aim was to compare the affinity with that of mere admixtures of the ingredients, LDH and PAC. The volatile compounds were dimethyl disulphide (CH3)2S and diethyl disulphide (C2H₅)2S). They are primarily responsible for toilet malodour and bad breath. Head Space-Gas Chromatography technique (HS-GC) was used to measure the concentration of these volatile compounds, which was found by determining the area under the peak (peak area).

A material with more affinity (adsorptivity) for the malodorous compounds would adsorb more of the volatile compounds. That would lower the head-space concentration of the malodourous compounds, which in turn would manifest itself in the form of a lower area under the peak. Conversely, a material with lesser affinity (adsorptivity) for the malodorous compounds would adsorb less of the volatile compounds. That would increase the head-space concentration of the malodourous compounds, which in turn would manifest itself as more area under the peak.

The results of Example 1 are shown in tabular form in Table 1 . Table 1

The data in Table 1 indicates clearly that the Composites are significantly better than their corresponding admixtures.

Example 2: Adsorption capacity of LDH-PAC composite v/s LDH-GAC composite In this example, the adsorption capacity of LDH-PAC (powder activated carbon) composite made in accordance with the present invention is compared to comparative composites made of LDH-GAC (granular activated carbon).

The composite of LDH and GAC was prepared by replacing PAC with GAC in the procedure described under the heading of Method in the Examples section.

The procedure of Example 1 (HS-GC) was followed to determine and compare the adsorptivity of both the composites.

The results of Example 2 are shown in tabular form in Table 2.

Table 2

The data in Table 2 indicates clearly that the Composite made in accordance with the invention is significantly better than a composite comprising GAC (Supplier: Active Carbon Ltd, India; particle size 500 μηη to 1000 μηη) and LDH. Example 3: Effect of concentration of LDH and PAC on porosity

In this example, the process described earlier was employed to prepare various composites within the scope of the present invention. The composites contained varying amounts of LDH and PAC. The porosity of all composites was determined by Mercury Porosimeter (Auto Pore® IV made by Micromeritics USA).

The data is presented in tabular form in Table 3.

Table 3

Porosity is a measure of how much can a substance adsorb of another substance. More porosity indicates more pores available for adsorbing the other substance. In other words it means that a material with more porosity is more likely to act a better adsorbent than a material with lower porosity.

The data in the Table 3 shows that porosity of the Composites A and B is high.

Claims

1. A composite adsorbent comprising nanoparticles of a layered double hydroxide (LDH) and a powder activated carbon (PAC) of the general formula [M²⁺i-_xM³⁺ _x (OH)2r(Xⁿ-)_q/n-yH20,in which,

M³⁺ = Al³⁺ or Fe³⁺;

x = [M³⁺]/ [M³⁺+M²⁺];

q = x

n = 1 to 4; and,

y= number of water molecules present on the LDH;

(b) adding an alkali to the dispersion of step (a);

(c) aging the dispersion of step (b) by heating it to 80 to 100°C;

2. A composite according to claim 1 wherein particle size of said powder activated carbon is 50 μηη to 300 μηι.

3. A composite according to any preceding claim 1 to 2 wherein amount of powder activated carbon is 70 to 90% by weight.

4. A composite according to any of claims 1 to 3wherein surface area of said

composite is 750 to 1000m²/g.

5. A composite according to any of claims 1 to 4 wherein molar ratio of M³⁺ to M²⁺ is 1 :2 to 1 :4.

6. A composite according to claim 5 wherein said ratio is 1 :3.

7. A composite according to any of claims 1 to 6 wherein M²⁺ is Mg²⁺, Cu²⁺ or Zn²⁺.

8. A hydrothermal process for preparing a composite adsorbent comprising

nanoparticles of a layered double hydroxide (LDH) and a powder activated carbon (PAC) of the general formula [M²⁺i_-xM³⁺x (OH)₂]^q+(X^n")_q/n-yH20, in which, M²⁺= Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺;

M³⁺ = Al³⁺ or Fe³⁺;

x = [M³⁺]/ [M³⁺ + M²⁺];

q = x

n = 1 to 4; and,

y = number of water molecules present on the LDH;

comprising, in sequence, the steps of:

(b) adding an alkali to the dispersion of step (a);

(c) aging the dispersion of step (b) by heating it to 80 to 100 °C;